Understanding the development and spatial distribution of alluvial patches in mixed bedrock-alluvial rivers is necessary to predict the mechanisms of the interactions between sediment transport, alluvial cover, and bedrock erosion. This study aims to analyze bedrock alluviation patterns using a 2D morphodynamic model, and to use the model results to better understand the mechanisms responsible for alluvial patterns observed experimentally. A series of simulations are conducted to explore how alluvial patterns in mixed bedrock-alluvial channels form and evolve for different channel slopes and antecedent sediment layer thicknesses. In initially bare bedrock low-slope channels, sediment cover increases with increasing sediment supply because areas of Froude-subcritical flow enable sediment deposition, while in steep-slope channels the flow remains fully supercritical and the model predicts so-called runaway alluviation. For channels initially covered with sediment, the model predicts a slope-dependent sediment supply threshold above which sediment cover increases with increasing sediment supply, and below which the bedrock becomes fully exposed. For a given sediment supply, the fraction of bedrock exposure and average alluvial thickness converge toward the equilibrium value regardless of the initial cover thickness as long as it exceeds a minimum threshold. Steep channels are able to maintain a continuous strip of sediment under sub-capacity sediment supply conditions by achieving a balance between increased form drag as bedforms develop and reduced surface roughness as the portion of alluvial cover decreases. In lower-slope channels, alluvial patches are distributed sporadically in regions of the subcritical flow.

中文翻译：

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基岩-冲积混合河道的冲积模式：2. 冲积斑块形成的控制

了解基岩-冲积混合河中冲积斑块的发育和空间分布对于预测泥沙输送、冲积覆盖和基岩侵蚀之间相互作用的机制是必要的。本研究旨在使用二维形态动力学模型分析基岩冲积模式，并利用模型结果更好地理解实验观察到的冲积模式的机制。进行了一系列模拟，以探索不同河道坡度和前期沉积物层厚度下混合基岩-冲积河道中的冲积模式如何形成和演化。在最初裸露的基岩低坡河道中，沉积物覆盖随着沉积物供应的增加而增加，因为弗劳德亚临界流区域使沉积物沉积，而在陡坡河道中，流动仍然完全超临界，并且模型预测了所谓的失控冲积。对于最初被沉积物覆盖的河道，该模型预测了一个与坡度相关的沉积物供应阈值，高于该阈值沉积物覆盖率随着沉积物供应的增加而增加，低于该阈值则基岩完全暴露。对于给定的沉积物供应，只要超过最小阈值，无论初始覆盖厚度如何，基岩暴露部分和平均冲积厚度都会向平衡值收敛。通过在床形发展时增加的形状阻力与冲积层部分减少时减少的表面粗糙度之间实现平衡，陡峭的河道能够在亚容量沉积物供应条件下保持连续的沉积物条带。在坡度较低的河道中，冲积斑块零星分布在亚临界流区域。